1. Field of the Invention
This invention relates generally to the fabrication of semiconductor devices, and, more particularly, to a method for forming a retrograde impurity profile in a semiconducting material.
2. Description of the Related Art
Ion implantation processes are widely used in the fabrication of very large scale integration (VLSI) and ultra large scale integration (ULSI) integrated circuit devices to introduce a desired quantity of dopant atoms into a target substrate material. For example, ion implantation is a commonly used process for introducing impurities, e.g., dopant materials, into the surface of a semiconducting substrate in order to modulate the conductive properties of the semiconducting substrate.
It is generally desired that a dopant material be implanted to some depth within a semiconducting substrate that is being prepared for device fabrication rather than having the dopant concentrated at or near only the surface of the semiconducting substrate. In one approach for introducing dopant impurities into a semiconducting substrate, a suitable dopant is introduced into the surface of the substrate and diffused to a desired depth by heating the substrate. Under such conditions, the diffusion of the dopant material occurs laterally as well as vertically, resulting in lateral spreading of the dopant within the substrate. Such lateral spreading of the dopant material has the undesired effect of reducing the packing density of the implanted material in the semiconducting substrate, which can adversely influence the properties of semiconducting devices subsequently fabricated on the surface of the substrate. Moreover, as illustrated in diffused impurity profile 6 of FIG. 1, although dopant can be diffused to some depth within the semiconducting substate, the dopant concentration is invariably highest at the surface of the substrate and decreases with increasing depth.
For many applications, it is desired that the peak concentration of the dopant material occurs at some depth under the surface of the substrate rather than at the substrate surface. This is generally referred to as a retrograde impurity profile. Retrograde impurity profiles may be formed using high-energy ion implantation processes, which allow for improved impurity depth control and reduced spreading compared with diffusion techniques. In an illustrative retrograde impurity profile 8, as represented in FIG. 1, the concentration Y of the implanted dopant material at depth X is higher than the concentration Z observed at depth X for diffused impurity profile 6. Thus, in contrast to diffused impurity profile 6, the concentration of the dopant material for a retrograde impurity profile 8 is highest at some depth within the substrate, rather than at the surface, and decreases as it approaches the substrate surface. Retrograde impurity profiles in a semiconducting substrate are particularly useful in integrated circuit manufacturing, for example, to form n-well and p-well regions in complementary metal oxide semiconductor (CMOS) transistors and to produce buried collector regions in bipolar transistors. Some of the advantages provided by retrograde impurity profiles in the manufacture of CMOS and other devices include reduced susceptibility to vertical punchthrough and improved protection from latchup.
In prior art methods for forming retrograde impurity profiles, dopant ions are commonly directed at a layer of material overlying the surface of the semiconducting substrate rather than being implanted directly into the substrate. As a result, the ions contact and penetrate this surface layer before they penetrate the semiconducting substrate. The surface layer may serve as a protective screen against contamination by metals or other impurities during the implant. Moreover, the layer may also be important in reducing the extent of ion channeling in the substrate by randomizing the direction of the ions as they enter the substrate lattice. This surface layer typically is referred to as a "sacrificial" layer because it is generally removed at some point after the implantation process.
The thickness of sacrificial surface layers used in prior processes for forming retrograde impurity profiles in semiconducting substrates has been generally greater than 200 .ANG.. However, sacrificial layers having thicknesses greater than 200 .ANG. may be associated with less than desirable dopant depth control and spreading, which can adversely effect the performance of transistors fabricated thereafter above the surface of the semiconducting substrate.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.